1. Field of the Invention
The present invention relates to an optical interconnection circuit board and a method of manufacturing the circuit board. More particularly, the invention relates to an optical interconnection circuit board having an optical waveguide for optically connecting different LSIs at high speeds and to a method of manufacturing the circuit board.
2. Description of the Related Art
Recently, electronic devices as bipolar transistors and field-effect transistors have been improved in terms of their performance. The clock frequency of the LSI has increased so that the LSI may operate at a remarkably high speed. However, the operating speed of a printed circuit board with LSIs mounted thereon is designed to operate at a speed lower than the operating speed of each LSI. The operation speed of the rack on which the printed circuit board is mounted is suppressed to be even lower. This is because the clock frequency must be lowered to prevent the deterioration of signals. In this regard, it should be noted that the transmission loss in the wires, the signal noise and the electromagnetic trouble increase as the operation clock frequency increases. To reduce the deterioration of the signal it is necessary to lengthen the wires that connect the LSIs. In view of this, the operation frequency cannot be raised as high the internal operation clock frequency of the LSI. Even if the LSIs, which are active devices, can operate high speeds, there is no way but decreasing the operating speed of an electronic wiring module.
In views of the aforementioned problem of the electronic wiring module, several optical connection modules for connecting the LSIs have been proposed. The optical connection module virtually has no frequency dependency, in terms of loss in a frequency region from DC to tens gigahertz or more. It has no electromagnetic troubles or causes no noise even if the ground potential change in the wiring path. Therefore, the optical connection module can transmit signals at tens of Gbps is possible. The optical connection module can operate at a very high speed on printed circuit boards or on the rack holding a printed circuit board.
To provide such an optical connection module, it is necessary to connect LSIs by optical waveguides or optical fibers. Generally, the optical connection in an optical connection module is achieved by aligning and connecting substrates, each having a flat optical waveguide, or by aligning and connecting optical fibers. Since it takes much time to align the substrates or optical fibers, the productivity of the optical connection module is inevitably lower that that of the electronic wiring module that comprises LSIs. To mount an electronic element, such as a semiconductor laser, a photodiode or the like, on an optical connection module, a notch is made in the optical waveguide and the electronic element is set in the notch. The position of the notch and the kind of the element set in the notch differ from module to module. This means that optical connection modules need to be designed, one by one.
The substrate of the optical connection module is generally flat. The light input/output part therefore extends at right angels to the substrate. An optical-path angle converter for changing the optical path by 90° is provided in the optical waveguide, and the optical components are mounted on the substrate. However, no optical-path angle converters have been realized, which are highly productive and efficient.
Hitherto, various methods have been practiced to mount the optical components. In one method, the optical waveguide is mechanically cut at 45° to reflect a light beam at right angles. In another method, a 45°-mirror is mounted at an output port of the optical waveguide. The method of cutting the optical waveguide at 45° can be used, but only at the ends of the connection module. A method of cutting the optical waveguide obliquely by dicing or laser process is generally employed. Another method of grinding the optical waveguide obliquely for each substrate is well known.
Of the methods of providing the 45°-mirrors, the method of aligning micro 45°-mirrors with an optical terminal is the simplest. To reduce mechanical arranging process, a new method has been proposed. In this method, the optical waveguide board and the micro mirrors are produced separately and bonded together. (See, for example, Society of Photo-Optical Instrumentation Engineering: SPIE Vol. 3288, p. 79).
FIG. 1 shows the optical waveguide board provided with micro mirrors described in the above document. As FIG. 1 shows, the micro 45°-mirrors 3 are provided on the mirror board 1. Note that reference numeral 15 denotes an optical waveguide board, reference numeral 4 designate an optical waveguide core layer, and reference numerals 2 and 5 represent clad layers. In FIG. 1, the arrows indicate the direction in which optical signals travel. Optical devices, LSIs and the like are mounted on the substrate 15 and electrically connected.
In these methods, an optical beam can undergo orthogonal transform. With the method of cutting the optical waveguide, however, it is difficult to form an optical input/output portion at any position and in any direction in the substrate. If many optical input/output portions are provided, the productivity will decrease. In the method of mounting external mirrors, not only the substrate becomes complicated, but also mounting failure or pollution is likely to occur because these mirrors are mechanically mounted. The problems described above hamper an increase in the productivity of the optical connection modules, rendering it difficult to manufacture highly general-purpose optical connection modules at low cost.
Jan. Pat. Appln. KOKAI Publication No. 2000-114581 discloses a method, in which active devices, such as semiconductor laser, photo diode and driving IC, are buried in the optical interconnection circuit board. The active devices are electrically connected to an electric wiring substrate. The electric-wiring substrate is mounted on the optical connection board. Another method is proposed in Society of Photo-Optical Instrumentation Engineers: SPIE Vol. 3288, p. 133. In this method, the substrate is made rough at one surface to deform its lower clad layer, and a mirror is mounted on the deformed portion.
In the first-mentioned method, the LSI is mounted by ordinary electric connection. An ordinary mounting scheme such as soldering reflow can therefore be applied. However, the active devices are buried in the mounting substrate, the structure can hardly enlarged to tens of centimeters due to thermal distortion, and wiring for the mounting is not suitable for general purpose. In the second-mentioned method, the optical waveguide and electric wiring are formed on the optical connection board, without performing any other treatment. The structure can be easily enlarged, and its mounting wiring pattern can be altered by changing the photolithography mask to another.
FIG. 2 shows the structure based on Society of Photo-Optical Instrumentation Engineers: SPIE Vol. 3288, p. 133. In FIG. 2, reference numeral 1 designates a substrate, reference numeral 2 a cladding layer of the optical waveguide, reference numeral 4 a core layer of the optical waveguide, reference numeral 5 a cladding layer of the optical waveguide, and reference numeral 3 denotes a buried mirror provided directly in contact with the optical waveguide core layer 4. As shown in FIG. 2, the substrate 1 and the cladding layer 2 have an uneven surface. The buried mirror 3 is formed by vapor-depositing metal on a slope corresponding to the border of the unevenness of the cladding layer 2. With this conventional structure, the light wave traveling through the optical waveguide is partially picked up. In the structure of FIG. 2, the intermediate terminal of optical bus functions as an optical tap. Here, the mirror 3 functions as an optical split mirror.
In this conventional structure, the height of the split mirror 3 is less than the thickness of the optical waveguide core layer 4. The split mirror 3 is buried in the optical waveguide core layer 4. Therefore, the mirror 3 splits a part of guided light wave, and the remaining part of the light wave can be distributed to a following optical waveguide to guide the light wave from the split mirror 3. Since the optical waveguide is a multi-mode waveguide, the light wave split by the mirror 3 can mix with the light wave passing over the mirror 3. Hence, optical distribution can be designed easily by simulating the traveling of the light wave. Further, the light wave thus guided can be input to the optical split portion from outside. The basic function as a signal bus can therefore be realized with a simple structure.
However, the substrate 1 and the lower clad layer 3 have an uneven surface in this conventional structure. The core layer 4 inevitably has an uneven surface. The energy loss in the light wave traveling through the optical waveguide is large. That is, the guided light wave produces a mode transformation loss at the border of the unevenness or at a bent portion of the core layer 4. The loss is greater than in a flat optical waveguide. This conventional structure is disadvantageous in that the energy loss in the guided light wave is too large. Since the buried mirror 3 is designed to achieve partial splitting, it, used as an optical path transformation mirror for optical transmission, reduces the opening area of a light incident section to a value less than the sectional area of the optical waveguide. Assume that vertical splitting efficiency of the buried type mirror 3 is 10% ( 1/10 split). Then, the sectional area of the optical transmission beam needs to be 1/10 the sectional area of the optical waveguide.
Generally, converging light beam results in an increase in the divergent angle of the beam. In view of this, it is no use to converge the beam so much that an angle exceeds a numeral aperture of an optical waveguide. Here arises the problem that optical coupling efficiency of the optical transmission portion may fall in some cases. This problem cannot help but be accepted as necessity. Nonetheless, it leads to an energy loss in the optical transmission of a one-way distribution of signal. Further, at an end portion of the optical waveguide, the guide light wave other than split light may emerge from the waveguide or may be reflected in an opposite direction. In this case, crosstalk will occur with other optical waveguide, or multi-reflection will take place in the optical waveguide. This generates a delay noise signal.
In the conventional structure described above, the split mirror or the buried mirror 3 is inclined due to deformation of the substrate. It is therefore required that the unevenness be made in the substrate and, hence, in the lower clad layer, too. This structure imposes restrictions on the material, size and the like of the substrate material. Consequently, the process of controlling the unevenness and the depth therefore is complicated, and the manufacturing method is difficult to control. To avoid this problem, the substrate may be made by molding. This method can indeed applied to mass-produce the same substrates. However, it requires a large initial investment for the mold, and the wiring pattern cannot be altered easily. In other words, this method is not a general-purpose one.
As described above, in the optical connection board for optically connecting LSIs, a highly practical 90°-optical path transform mechanism has not been yet realized. The productivity cannot be raised, and the manufacturing cost cannot be lowered.